Abstract

We have established a method to quantify and optimize the plasmonic behavior of aluminum thin films by coupling spectroscopic ellipsometry into surface plasmon polaritons using a liquid prism cell in a modified Otto configuration. This procedure was applied to Al thin films deposited by four different methods, as well as to single crystal Al substrates, to determine the broadband optical constants and calculate plasmonic figures of merit. The best performance was achieved with Al films that have been sputter-deposited at high temperatures of 350°C, followed by chemical mechanical polishing. This combination of temperature and post-processing produced aluminum films with both large grain size and low surface roughness. Comparing these figures of merit with literature values of gold, silver, and copper shows that at blue and ultraviolet wavelengths, optimized aluminum has the highest figure of merit of all materials studied. We further employ the Ashcroft and Sturm theory of optical conductivity to extract the electron scattering times for the Drude and effective interband transitions, interband transition energies, and the optical mass of electrons.

Modeled Ψ and Δ curves for the layered structure shown in Fig. 1 using the materials constants listed in Section 2 and aluminum values from the WVASE32® database, (a). The vertical dashed line corresponds to the minima in reflection spectra resulting from coupling to surface plasmons on an aluminum sample. In (b), experimentally measured Ψ and Δ curves for the layered structure shown in Fig. 1 with the room temperature sputtered sample.

Ψ and Δ curves for the five aluminium samples measured. Experimentally measured data is shown as open circles and model fits are shown as solid lines. The vertical, dashed black line corresponds to the wavelength of strongest coupling. Measurements were performed from 69° to 71° for each sample, and the curve shown represents the angle of strongest SPR coupling.

For each sample, the real and imaginary parts of the permittivity are shown in (a) and (b), respectively. The figure of merit for the five aluminum samples is plotted in (c). In (d), the high temperature sputtered and CMP’d aluminum film is compared with the Johnson and Christy data for gold, silver, and copper [23].

Tables (3)

Table 2 Modeling parameters used in the ellipsometry analysis of each aluminum sample. In the bottom row, the terms D, T-L, and L refer to the Drude, Tauc-Lorentz, and Lorentz oscillators needed to accurately model each sample.

Table 3 Optical properties for the five aluminum samples extracted by fitting the ellipsometry data in Fig. 4 to the Ashcroft and Sturm model of optical conductivity. For comparison, the last row lists grain size measurements from Table 1, and the last column lists experimental and theoretical predictions in the literature. The order in which the numbers are listed corresponds to the order in which the references are listed.

Metrics

Table 1

AFM analysis of the five aluminum samples studied.

Crystal

CMP

MBE

RT Sputtered

Evaporated

RMS Roughness (nm)

2.1

1.80

0.43

1.63

6.9

Median Grain Diameter (nm)

> 100, 000

1438

939

124.2

52.2

Table 2

Modeling parameters used in the ellipsometry analysis of each aluminum sample. In the bottom row, the terms D, T-L, and L refer to the Drude, Tauc-Lorentz, and Lorentz oscillators needed to accurately model each sample.

Crystal

CMP

MBE

RT Sputtered

Evaporated

RMS Roughness (nm)

2.1

1.80

0.43

1.63

6.9

Intermix Thickness (nm)

2.10

2.45

0

0

3.32

Intermix Fraction (%)

25

25

0

0

25

Oscillators

D,T-L,L,L

D,T-L,L,L

D,T-L,L

D,T-L,L

D,T-L,L

Table 3

Optical properties for the five aluminum samples extracted by fitting the ellipsometry data in Fig. 4 to the Ashcroft and Sturm model of optical conductivity. For comparison, the last row lists grain size measurements from Table 1, and the last column lists experimental and theoretical predictions in the literature. The order in which the numbers are listed corresponds to the order in which the references are listed.

Tables (3)

Table 1

AFM analysis of the five aluminum samples studied.

Crystal

CMP

MBE

RT Sputtered

Evaporated

RMS Roughness (nm)

2.1

1.80

0.43

1.63

6.9

Median Grain Diameter (nm)

> 100, 000

1438

939

124.2

52.2

Table 2

Modeling parameters used in the ellipsometry analysis of each aluminum sample. In the bottom row, the terms D, T-L, and L refer to the Drude, Tauc-Lorentz, and Lorentz oscillators needed to accurately model each sample.

Crystal

CMP

MBE

RT Sputtered

Evaporated

RMS Roughness (nm)

2.1

1.80

0.43

1.63

6.9

Intermix Thickness (nm)

2.10

2.45

0

0

3.32

Intermix Fraction (%)

25

25

0

0

25

Oscillators

D,T-L,L,L

D,T-L,L,L

D,T-L,L

D,T-L,L

D,T-L,L

Table 3

Optical properties for the five aluminum samples extracted by fitting the ellipsometry data in Fig. 4 to the Ashcroft and Sturm model of optical conductivity. For comparison, the last row lists grain size measurements from Table 1, and the last column lists experimental and theoretical predictions in the literature. The order in which the numbers are listed corresponds to the order in which the references are listed.